Abstract
Bck1, Mkk1 and Slt2 are three mitogen-activated protein (MAP) kinases constituting cell wall integrity (CWI) pathway that may control multi-stress responses via crosstalk with high-osmolarity glycerol (HOG) pathway in budding yeast. In this study, Bck1, Mkk1 and Slt2 orthologues in Beauveria bassiana were confirmed as the three-module cascade essential for CWI because cell wall impairment occurred in the hyphae and conidia of Δbck1, Δmkk1 and Δslt2 examined in multiple experiments. Strikingly, all the deletion mutants became more sensitive to hyperosmotic NaCl and sorbitol with the Western blot of Hog1 phosphorylation being weakened in Δbck1 and absent in Δmkk1 and Δslt2. Apart from crossing responses to cell wall perturbation and high osmolarity, three deletion mutants exhibited faster growth and conidiation on nutrition-rich medium, much less virulence to Galleria mellonella larvae, and higher sensitivity to nutritional, fungicidal, thermal and UV-B irradiative stresses, accompanied with less accumulation of intracellular mannitol and trehalose. Moreover, Δmkk1 and Δslt2 were equally more sensitive to all the stresses of different types except wet-heat stress than wild type and more or less different from Δbck1 in sensitivity to most of the stresses despite their null responses to two oxidants. All the changes in three deletion mutants were restored by each targeted gene complementation. Taken together, the CWI-required Bck1, Mkk1 and Slt2 are all positive, but differential, regulators of multi-stress tolerance and virulence perhaps due to interplay with the HOG pathway essential for osmoregulation, thereby contributing greatly to the biocontrol potential of the fungal entomopathogen.
Highlights
Mitogen-activated protein (MAP) kinases in three-module cascades (MAPKKK/MAPKK/MAPK) are a family of serinethreonine proteins which constitute the signaling pathways regulating mating, filamentation growth, high-osmolarity glycerol (HOG), cell wall integrity (CWI), spore wall assembly, and HOGlike Spc1 in eukaryotes [1,2,3]
The cascades hallmarked by the MAPKs Slt2 and Hog1 are two stress-responsive pathways mainly responsible for the regulation of CWI and osmotolerance respectively
We found that Slt2 and Mkk1 played equal roles in regulating cellular responses to nutritional, cell wall perturbing, osmotic, fungicidal, and UV-B stresses but to some degree differed from Bck1 in mediating most of the examined stress responses
Summary
Mitogen-activated protein (MAP) kinases in three-module cascades (MAPKKK/MAPKK/MAPK) are a family of serinethreonine proteins which constitute the signaling pathways regulating mating, filamentation growth, high-osmolarity glycerol (HOG), cell wall integrity (CWI), spore wall assembly, and HOGlike Spc in eukaryotes [1,2,3]. The cascaded kinases can be activated by developmental and environmental cues through the dual phosphorylation of their threonine and tyrosine residues that are functionally conserved [4]. The cascades hallmarked by the MAPKs Slt and Hog are two stress-responsive pathways mainly responsible for the regulation of CWI and osmotolerance respectively. The CWI-regulated cascade can mediate multi-stress responses via crosstalk with HOG partners in Saccharomyces cerevisiae [5,6,7]. The Hog orthologue Sak in Botrytis cinerea was phosphorylated upon cell wall challenging and interfered with the phosphorylation status of the Slt orthologue Bmp3 [8]
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